The continuing development and widespread implementation of wireless/radio communication systems, such as wireless PAN (personal area network), wireless LAN (local area network), wireless WAN (wide area network), cellular networks, etc., is driving the market demand for high-performance, highly-integrated and low-power, low cost solutions for on-chip radio communication systems that operate at millimeter-wave frequencies. For millimeter wave applications, integrated devices (e.g., integrated transmitter, receiver, transceiver systems) can be fabricated using GaAs or InP semiconductor technologies, as such technologies can provide the speed and power that is needed for such applications.
Wireless communication systems include transmitter circuits that typically employ a power amplifier circuit to output transmission signals at a required transmission power level to an antenna load. Moreover, automated level control (ALC) circuits are commonly employed in wireless communication systems to provide closed loop control of transmission power level for various purposes such as regulating transmission power to ensure compliance with regulations imposed on RF emissions, maintain constant transmitter output power over temperature or process variations, etc.
In general, conventional closed loop ALC systems are implemented using power detectors to sample a portion of the transmitter output power level and convert the sampled power to a DC voltage that provides an indication of the output power level. By way of example, some conventional power detector circuit designs are implemented using a detector capacitor to capacitively couple a portion of transmission power from a transmission line to the input of a power detector circuit that includes a diode (e.g., PIN diode) to rectify the coupled power and an RC filter to filter the rectified signal and output a constant voltage signal proportional to the transmitter output power level. The output of the power detector circuit can processed using various techniques known in the art for closed loop control of transmission output power.
FIG. 1 is a generic schematic illustration of a conventional power detection method using a detector capacitor to capacitively couple a portion of transmission power from a transmission line to the input of a power detector circuit. FIG. 1 illustrates a portion of a transmission path for transmitting high-frequency power from a power source (e.g., power amplifier) to a load (e.g., antenna). The transmission path comprises a power detector (10) comprising a detector capacitor (12) which is interposed at a point along the transmission path between transmission lines (11A) and (11B). The transmission lines (11A, 11B) are depicted in their equivalent circuit model of a lossless transmission line having distributed capacitance Co and inductance Lo per unit length.
In conventional designs, the detector capacitor (12) can be used with a resistor to tap the voltage waveform of the signal at some point along the transmit path. For high frequency applications (microwave, millimeter wave) where the transmission lines 11A and 11B are on-chip microstrip lines or other printed transmission lines, the use of the detector capacitor (12) for capacitively coupling power to the detector circuit is problematic.
For instance, the size of the detector capacitor that is needed to provide a required coupling capacitance for a given application can pose practical limits on integration density for integrated millimeter wave systems. Moreover, assuming the transmission lines are designed for a given characteristic impedance ZO (e.g., 50 Ohms), the introduction of the detector capacitor (12) in the transmit path between the transmission lines (11A, 11B) produces an impedance discontinuity (impedance mismatch between the coupling capacitance of detector capacitor (12) and the characteristic impedance of the transmission lines) that can result in high insertion loss, decreased coupling efficiency, and lead to errors in power detection measurements.
To compensate for such impedance mismatches, a matching network may be used to match the impedance of the power detector (10) to the characteristic impedance ZO of the transmission lines (11A, 11B). For microwave and millimeter wave applications, however, the impedance matching networks for power detector can be large in size (thus limiting integration density) and ineffective for efficient power detection under variable loads.